Theory of Atmospheric Currents. 17 
one component of the force due to the earth's rotation," which 
Mr. Ferrel and Professor Everett have (independently as I 
understand it) discovered. " If he had taken in this latter com- 
ponent also, and resolved it in the direction of the line of 
motion and perpendicular to it, as he did the former he would 
have found that the parts in the direction of the motion arising 
from both components exactly cancel one another n all cases, 
and that the resultant of both components is a force perpendi- 
cular to the direction of motion," — which of course "tends only 
to change the direction of the motion and never to accelerate or 
retard it, in whatever direction it may be." 
So that, on Mr. Ferrers own showing, a body set in relative 
motion, towards the pole or in any other direction, will, after 
all, neither be accelerated nor retarded, in virtue of the " prin- 
ciple of conservation of areas," but will maintain its velocity 
and be deflected from (that is, never come near to) the pole. 
In truth it will, when supposed free in its motion, move 
equably in a great circle, as I have already explained, in oppo- 
sition to Mr. Ferrel, who, not here only but in Silliman's 
Journal, explicitly denies it *. For these two forces which 
combine to "deflect" the moving body (that is, to alter the 
apparent direction of its motion) are the merely ''hypothe- 
tical" forces which, I have above said, are introduced by Mr. 
Ferrel himself — justifiably enough, if he understood them to 
be such. To explain — 
If a body moving uniformly in a straight line is viewed 
from an observatory itself in motion, it will appear to be 
moving in some curve and with varying velocity. And the 
observer may express this fact by saying that, relatively to 
him, it moves as if acted on by such and such forces. And 
this is our case. We are carried along by the earth's rotation 
from west to east; and if a mass is set moving, in space, 
equably along a great circle, it moves, as we estimate it, away 
from our latitude and our longitude, and unequably. "Great- 
circle sailing " is really straight sailing ; but I dare say that 
many an uneducated steersman, when following the rules 
given him for that operation, thinks he is altering the straight 
course. I should here add that Mr. Ferrel does, in Silliman's 
Journal, when speaking of the resultant "force," remark that 
it " is not an absolute force, but relative ; somewhat in the 
nature of centrifugal force." But this seems to have been a 
fleeting thought not duly followed out ; or he would not, a 
dozen years later, have written what I have quoted, 
I do not know what Professor Everett has written, or 
* " If a body 'were set in motion upon the surface of the earth, it would 
not in general move in the circumference of a great circle." 
Phil. Mag. S. 5. Yol. 16. No. 97. July 1883. C 
